In the glow plug of the prior art, as shown in FIG. 4, a heater tube 2 and a metal shell are jointed to each other by press-fitting the former in the latter. Specifically, the heater tube 2 is set in a metal shell 4 having an internal bore 4a, and the heater tube 2 is inserted so far into a recessed holding portion 001a formed in a holding tool 001 that the closed leading end face of the heater tube 2 abuts against the holding portion 001a. Then, the heater tube 2 is press-fitted by applying an axial load to the holding tool 001 by a hydraulic press or the like.
As disclosed in JP-A-11-94254, on the other hand, the a press-fitting load is applied to the heater tube to press-fit its portion in the fitting portion formed in the internal bore of a housing, by clamping the outer circumference of the heater tube with a recessed collet and by pressing the collet.
However, the heater tube to be used in the glow plug is generally swaged so that its external diameter is easily dispersed according to the working condition. Moreover, the internal diameter of the fitting portion, which is formed in the internal bore of the metal shell to press-fit the press-fitted portion of the heater tube, is also easily dispersed according to the cutting condition or the like. As a result, the load necessary for the press-fit is also dispersed. In case the press-fit is done by the method shown in FIG. 4, therefore, the heater tube 2 may be bent (as will also be called “buckled”). After press-fitted, therefore, the heater tube 2 may be unable to be removed (as will also be called “bitten by”) from the holding portion 001a of the holding tool 001.
On the other hand, a method disclosed in JP-A-11-94254 can inhibit the buckling to some extent but is insufficient for coping with the dispersion in the load necessary for the aforementioned press-fit. As a result, a high pressure is applied to the side face of the heater tube. Therefore, the side face of the heater tube is highly probably collapsed, deformed and damaged so that the oil in the heater tube is short-circuited to raise a problem that the internal resistance is changed.
Therefore, the dispersion in the external diameter of the heater tube is inhibited by cutting or grinding the heater tube. However, there arises another problem that the working steps increase and become troublesome.